In-mould electronics (IME) promises to enable high-volume production of structural electronics where the electronic circuitry and functionality are part of the 3D-shaped structure itself. This will save weight and space and will enable new elegant designs.

IME is not exactly a new process or technology. In fact, in many ways, it is an evolution of the well-established IMD, or in-mould decoration, in which moulding (or other ways of 3D forming) are combined with graphic printing. The transition from IMD to IME however is not straightforward, especially on a commercial scale. Indeed, this partially explains why it has taken this long for IME to establish lasting commercial success despite all the efforts and false starts.

This article draws from the above-mentioned report to outline key innovation trends underpinning the commercialisation of IME. Here, we will consider trends in materials, processes, as well as design. This will give the reader a better insight into this promising technology.

Everything must change to enable the commercialisation of IME. New materials must be developed that can survive new requirements such as stretching and 3D forming; new processes must be developed to combine 2D printing, 3D forming and rigid component placement; and new design procedures and product concepts must be developed based on material and process characteristics as well as market needs. This extensive change at many levels has prolonged the go-to-market timelines.

Material trends

Functional materials in IME must withstand new requirements. They must survive a one-off significant stretching event as the 2D printed sheet is formed into a 3D object. This is much more challenging to achieve for functional (vs mere graphical) inks since elongation can disrupt the function, eg, break the conduction path in conductive inks.

There is no single required degree of stretching; however, in general higher levels of stretchability are desired. As a crude rule of thumb, 20% elongation is the minimum whereas 60% or higher is in many cases preferred. Suppliers already seek to differentiate by the stretchability of the material since it eases process development and gives more design freedom.

Functional materials must also be reliable under harsh field conditions. This is critical particularly in automotive and similar applications. This aspect, surprisingly, was often neglected in the early days. Indeed, famous IME product failures and recalls have been caused by unreliability. The properties of utilised materials can often change significantly during high-humidity and high-temperature tests. This change should be factored into the design of the product.

IME is not composed of a single layer of materials. In fact, a stack of materials will need to be printed to achieve the required effect. This stack can include graphic inks, conductive inks, dielectrics, transparent conductive inks, carbon overcoats, and so on.

Thus far, the most studied functional material has been the conductive ink. Today, there are multiple suppliers across the world offering conductive inks for IME. This attention is justified because metal-filled (almost always silver) conductive inks represent the most expensive and high-value material in the stack and because they are the most sensitive to changes in the conduction, eg, elongation.

Other materials are also critical in the process. In particular, low-temperature printable conductive adhesives that also exhibit some stretchability are these days the subject of increased product offerings. In general, all functional materials must also be compatible with one another. This compatibility is critical especially during the forming process and significantly impacts final properties. Indeed, even the sequences in which the materials are deposited can have an impact. This is a development challenge but also an opportunity to develop and sell complete IME material portfolios.

The substrate also represents a development and supply opportunity. Most have thus far utilised a polycarbonate substrate due to its good formability; however, many are now developing alternative such as special PETs. This is a space to watch closely. The moulding material will also be important, especially if new material can be developed to relax the moulding conditions. This would ease performance requirements for all the other materials in the process.

Process trends and challenges

The process is critical. It is not straightforward. It involves printing and drying/curing multiple functional and graphical materials on a 2D formable substrate such as PC. It then involves converting the 2D sheet into the 3D shape via thermo or vacuum moulding under elevated temperatures. The overmoulding must then take place at high temperatures too. In many cases, it might be tempting to cut corners to streamline the process for mass production, but past experience suggests that this comes with significant perils.

The question of pick-and-placing rigid components is also challenging. If the pick-and-place occurs after forming, then the pick-and-place machine must be able to handle placement in a 3D space. This will require specialised pick-and-place as well as adhesive dispensing tools, and will almost certainly slow down the process. The pick-and-place could also occuron a 2D sheet prior to forming. This would however require special adhesives as well as careful product and process design to ensure that the rigid components will remain attached after all the forming steps.

In general, the process development is complex. It requires deep knowledge of the materials as well as all the process steps. The question of yield is a persistent and particular challenge. This is because defects cannot be repaired since electronics are embedded or structurally integrated. As such, defects are expensive since they waste the fully formed devices. In general, there is a steep learning curve to be travelled. This has created the need for centres or entities with accumulated know-how and expertise to cut down development time and technical barriers to entry. It has also meant that many traditional membrane switch or other functional printers with low-risk appetites and/or tight cash flows have had to wait for the industry to mature further before investing to evolve their business towards IME. This evolution will increasingly become inevitable and will accelerate as IME achieves a higher level of technical maturity and perhaps modularity.

Design trends and challenges

The design of IME products is also not straightforward. This is because it requires deep knowledge of material and process characteristics. It is not a streamlined process yet, lacking established software packages with drop-and-place component/functional libraries. This is in stark contrast to design processes found, say, in standard PCBs. The market requirements are also not clear-cut, well-established or convergent yet. This is because, despite years of development, the industry is still in an exploratory phase where it is developing numerous prototypes and running qualification processes. The products and prototypes are still mainly custom made without standard design.

These all complicate the product development process, prolong the time-to-market, and form barriers to entry for users as well as potential producers. However, the industry is responding now and some firms are positioning to fill exactly this need, thus helping accelerate overall commercialisation.

The themes briefly discussed in this article have certainly prolonged the time-to-market. Indeed, IME, despite its semblance of simplicity on paper, is a complex endeavour, requiring drastic changes in materials, processes, designs and product concepts. The industry however has come a long way in terms of its accumulated learning as well as product offering. Low-volume products are already on the market and multiple high-volume applications are not too far from final qualification. Indeed, we forecast this market to exceed $1 billion within the next decade.

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